1. Field of the Invention
The present invention relates to a liquid ejecting head which ejects a liquid such as ink from an ejection port.
2. Description of the Related Art
A recording apparatus provided with a liquid ejecting head can output characters and images of high quality at low cost. Nowadays it is desired to reduce the size of a droplet for enhancing an image quality, and it is known that a slight variation of the dimension of a nozzle gives influence on ejection and consequently gives influence on the image quality. While such a high image quality is required as described above, in a conventional method of bonding an orifice plate onto a silicon substrate, dimensional tolerances for the warpage in the upper and lower sides and front and back sides of an orifice plate, lack of bonding precision occasionally, and the like may give influence on ejection stability and ejection quantity. In order to achieve reduction in the size of the droplet and stable ejection for the further enhancement of the image quality, a resin lamination on a silicon substrate has become a mainstream as a method of producing the nozzle.
In the liquid ejecting head, a refilling period of time until the pressure chamber is filled again with ink after a predetermined amount of ink has been ejected from the pressure chamber (i.e. easiness of refilling) is dependent on physical properties of the ink and a structure of an ink flow channel. For instance, because a high viscosity ink has a high flow resistance in a region including a pressure chamber, the refilling period of time tends to be relatively long. In addition, when the sectional area of the ink flow channel is small, the flow resistance becomes large from a common liquid chamber to a pressure chamber, and accordingly the refilling period of time also becomes relatively long. The frequency (ejection frequency) with which one nozzle repeats ejection is required to have a longer cycle time than the refilling period of time which is determined by the physical properties of the ink and the structure of the ink flow channel.
On the other hand, when the refilling period of time is shorter than necessary compared to the cycle of ejection frequency, and flow resistance is small, a tip part (meniscus) of the ink in the ink channel occasionally overshoots when the ink has been refilled, and the ink occasionally overflows from the periphery of the ejection port. As has been described above, the ejection frequency in a recording element is required to be set at a short cycle of such a degree as not to cause overshooting.
In other words, the upper limit of the refilling frequency to be actually used is determined according to the physical properties of the ink and the structure of the ink flow channel, and it is desirable that the refilling frequency is set so as not to exceed this range.
However, when a carriage speed of an ink-jet printer is increased in an attempt to achieve further enhancement of the speed of the liquid ejecting head, another problem occurs which has not been considered up to now.
Specifically, as the carriage speed is increased, the refilling frequency of a head must necessarily be increased. As has been described above, in a head having high refilling frequency, the meniscus of ink overshoots when the ink is refilled, and the ink tends to easily overflow from the periphery of the ejection port.
Furthermore, when images are continuously recorded at high frequency, at a high printing speed, and with high duty for a long period of time, a large quantity of ink mist is produced, and an ink droplet gradually becomes stagnant on the face of the ejection port, which causes the ink to easily overflow from the ejection port. Still furthermore, when the water repellency of an ejection port forming face of a print head is lowered or the ejection port forming face is hydrophilic, ink tends to more remarkably overflow from the periphery of the ejection port due to the overshooting of the meniscus of the tip part of the ink when the ink has been refilled.
As a method of reducing an ink which overflows from the periphery of the ejection port, a multi-pass recording method is widely known, which reduces an effective frequency of the nozzle of the recording head. However, in the multi-pass recording method, when the carriage frequency is increased, the number of scanning times for recording prints increases, which accordingly results in a harmful effect on high-speed printing.
Specification of U.S. Pat. No. 7,585,616 discloses a nozzle having a recess part formed in an ejection port portion, as a nozzle effective in reducing the stagnation of an ink droplet on the face of the ejection port.
However, in the nozzle having the recess part formed on the face of the ejection port as is disclosed in the specification of U.S. Pat. No. 7,585,616, the ink droplet stagnates in the recess part instead of on the face of the ejection port. Furthermore, in the above described head having high refilling frequency, the meniscus at the tip part of the ink may disadvantageously overshoot when the ink has been refilled and the ink overflows from the recess part. The overflowed ink reaches even a neighboring nozzle and causes an ejection kink of the neighboring nozzle.